JOHANNESBURG, South Africa (AP) _ Police shot 12 blacks to death and at least 70 other people were injured in overnight clashes in Soweto, the government said today. A newspaper said a mob hacked one Soweto councilor to death.

There were unofficial reports from residents, including doctors and clergymen, that 20 people were killed and up to 100 people injured. The violence was the worst reported since a nationwide state of emergency was declared June 12.

Residents said the clashes were linked to efforts by Soweto council police to evict tenants conducting a rent boycott.

The state Bureau for Information said eight of the deaths and most of the injuries occurred Tuesday night when police shot into a crowd from which a hand grenade had been thrown. The grenade injured four police officers, three of them black and one white.

The bureau said the crowd of about 300 people had gathered at about 10 p.m. Tuesday at a flaming barricade outside a liquor store in the black township.

Four other deaths occurred early today when police fired at a group of about 80 people at another barricade, it said. Someone in the crowd fired at the police and injured a black officer, the bureau said, but did not specify if police had fired first.

''The council police are shooting left and right. They are shooting at everyone, everything,'' the Sowetan newspaper quoted an unidentified resident as saying.

The Sowetan said a mob hacked Soweto city councilor Sydenham Mkhwanazi to death, and that the house of another councilor, Sigfriend Manthata, was burned down.

A third councilor, Silas Tshabalala, was shot in the leg when his own guard apparently fired in panic, the newspaper said. Residents and other reporters confirmed the three incidents.

However, the state Bureau for Information said it could not confirm the Sowetan's reports. Asked whether unrest was continuing today in Soweto, the bureau said: ''The situation there is not abnormal.''

Sources at Soweto's Baragwanath Hospital told the South African Press Association that more than 80 people were treated for injuries.

The government claimed recently that anti-apartheid violence was subsiding since imposition of the state of emergency, under which most public gatherings are banned and police are empowered to detain people without charge.

Emergency rules bar journalists from revealing the names of detainees or publishing statements deemed subversive.

The shootings by police took place in one of Soweto's poorest neighborhoods, White City. Black reporters who were in contact with Soweto by telephone said violence was spreading to other neighborhoods.

They said schools were closed, and pupils who showed up were told to come back next month. The government said most Soweto students were staying away from classes but that no schools were officially closed.

Witnesses said youths set up barricades of horse carts and trash cans, hurled stones at police and went house to house asking other residents to join the conflict.

The Sowetan said the violence began late Tuesday as town council police evicted some families who were refusing to pay rent for houses owned by the government-supported council.

Another resident told the South African Press Association that the trouble started when police broke up a rent boycott meeting at about 7:30 p.m.

Anti-apartheid activist Winnie Mandela toured White City today. She told reporters many Soweto residents were very upset by the evictions and predicted the situation would deteriorate.

Mrs. Mandela is the wife of Nelson Mandela, a jailed leader of the African National Congress guerrilla group.

She called the violence ''an ominous sign of the times ahead.''

The confirmed toll of 12 people killed by police represented the deadliest confrontation between blacks and security forces since 19 people were killed March 21, 1985, during a march in the eastern Cape province township of Langa. There have been higher one-day death tolls in South Africa this year, but they involved clashes between rival black political and tribal factions.

Some reporters said about 500 people tried to march today on the town council office to protest evictions, but were dispersed by security forces firing tear gas.

The town clerk, Nico Malan, said police evicted four Soweto familes Tuesday but not in the areas where the violence occurred. He said only three other Soweto families have been evicted since the town council's deadline for rent payment expired at the end of July.

Rent boycotts have become widespread in Soweto only in the last three months. The boycotts began in other townships in September 1984 to protest rent increases.

Rioting also broke out then, beginning the current phase of anti-apartheid unrest. More than 2,100 people, most of them black, have died in nationwide unrest since then, including more than 255 since the state of emergency was imposed.

By law and custom, apartheid establishes a racially segregated society in which South Africa's 24-million black majority has no vote in national affairs. The 5-million white minority controls the economy and maintains separate districts, schools and health services.